Method and device for producing a compound semiconductor layer

ABSTRACT

In a method for producing a I-III-VI compound semiconductor layer, a substrate is provided with a coating which has a metallic precursor layer. The coating is kept, for the duration of a process time, at temperatures of at least 350 degrees C. and the metallic precursor layer, in the presence of a chalcogen at an ambient pressure of between 500 mbar and 1500 mbar, is converted into a compound semiconductor layer. The coating is kept at temperatures for the duration of an activation time which attain at least an activation barrier temperature, whereby as the activation barrier temperature a value of at least 600° C. is selected.

The present invention concerns a method for producing a compoundsemiconductor layer according to the preamble of claim 1, together witha device for carrying out the method according to the preamble of claim14.

Environmentally-friendly and inexpensive energy generation is a centralproblem nowadays. One approach to a solution for this problem is that ofcollecting photovoltaic current from sunlight by means of solar cells,or solar modules. The greater the conversion efficiency of the solarmodules and the lower their costs of manufacture, the lower the cost ofgenerating the current. Against this background, so-called thin-layersolar modules are a promising approach to a solution, since these can beproduced at little cost in terms of material and energy and also enablea good conversion efficiency, i.e. high grades of efficiency. Inparticular, thin-layer solar modules based on I-III-VI-compoundsemiconductors have proven their value. These include, for example,compound semiconductors consisting of copper-indium-selenide (CIS) orcopper-indium-gallium-selenide (CIGS).

Compound semiconductor layers can be manufactured in various ways, forexample by co-evaporation of the elements involved. Anothermanufacturing option involves so-called deposition-reaction-processes,in which, firstly, a metallic precursor layer is deposited and this issubsequently converted with a chalcogen into the respective compoundsemiconductor. One embodiment of such a deposition-reaction-process, inwhich the metallic precursor layer is thermally converted in a simpleand rapid continuous process into the compound semiconductor layer, isdescribed in patent document WO 2009/033674 A2.

In thin-layer solar cells, or thin-layer solar cell modules, efficiencyis influenced, inter alfa, by the homogeneity of the compoundsemiconductor layer used. It has emerged indeposition-reaction-processes that elements of the compoundsemiconductor, for example gallium, can spread out inhomogeneously overthe thickness of the compound semiconductor layer. This is especiallythe case when process times are short for technical production reasons.Said gallium builds up, for example, preferably on a back contact, whichis usually applied onto a substrate in the form of a metallic layer,before the metallic precursor layer is deposited onto this back contactand this is converted into a compound semiconductor. As a result of thisbuild-up on the back contact, the gallium is spread inhomogeneously overthe thickness of the compound semiconductor layer. FIG. 1 illustratesthis type of inhomogeneous distribution using the example of a CIGScompound semiconductor layer. The depth shown therein has its neutralpoint on the upper side of the compound semiconductor layer andincreases in its value towards the back contact arranged on thesubstrate, which in the illustration in FIG. 1 would be arranged on theright edge of the image. FIG. 1 thus illustrates a build-up of thegallium in the compound semiconductor layer on the back contact. Thiscan lead to a reduced open circuit voltage and hence to a reducedefficiency of the solar cell or the solar module produced from thiscompound semiconductor layer.

Against this background, the invention is based on the problem ofproviding a method by means of which a I-III-VI-compound semiconductorlayer can be produced economically and with a more homogeneousdistribution of the elements involved.

This problem is solved by a method with the features of claim 1.

The invention is also based on the problem of providing a device forcarrying out the method.

This problem is solved by a device with the features of claim 14.

Advantageous refinements are the subject matter of the respectivedependant sub-claims.

The method according to the invention makes provision that a substrateis provided with a coating which has a metallic precursor layer. Thecoating is then kept for the duration of a process time at temperaturesof at least 350° C. and the metallic precursor layer is converted, inthe presence of a chalcogen and at an ambient pressure of between 500mbar and 1500 mbar, into the compound semiconductor layer. The coatingis also kept for the duration of an activation time at temperatureswhich attain at least an activation barrier temperature. A value of atleast 600° C. is then selected as activation barrier temperature. It isespecially preferable for the metallic precursor layer to be convertedinto the compound semiconductor layer at an ambient pressure of between850 mbar and 1150 mbar.

The term I-III-VI-compound semiconductor layer is to be understood tomean a layer of a compound semiconductor which is formed from elementsfrom Group IB of the periodic table of the elements, for example copper,from elements of Group IIIA of the periodic table of the elements, forexample aluminium, gallium or indium, and at least one chalcogen fromGroup VIA of the periodic table, for example sulphur, selenium ortellurium.

In an especially preferred embodiment of the invention, the metallicprecursor layer is converted into a CIS- or CIGS compound semiconductorlayer.

The fact that the coating is kept at temperatures of at least 350° C. isnot to be understood to mean that the coating is heated to a definedtemperature and kept at this defined temperature throughout the entireprocess time. Instead, the invention makes provision that during theprocess time, the coating is at any temperature greater than or equal to350° C. The temperature of the coating can vary during the process time,but during the process time it is always at least 350° C.

The keeping of the coating at temperatures which reach at least anactivation barrier temperature is to be understood in the same way. Thecoating can thus take on any temperatures during the activation timewhich are greater than or equal to the activation barrier temperature.The temperature of the coating can vary during the activation time, butit is always equal to at least the activation barrier temperature.

A coating in the sense of the present invention can consist solely of ametallic precursor layer, for example a layer containing the metalscopper, gallium and indium, or contain further components, for example achalcogen layer, which is deposited onto the metallic precursor layer.The term of the coating thus encompasses, in the present invention, alllayers arranged on the substrate and the metallic back contact layerwhich may be provided there. The coating consists, after partialconversion of the metallic precursor layer, for example, partly of acompound semiconductor. The compound semiconductor layer which ispresent following complete conversion thus likewise represents a coatingin the present sense.

As already mentioned, a chalcogen, in particular selenium, can bedeposited as part of the coating onto the metallic precursor layer, inparticular by means of a physical deposition from the vapour phase atatmospheric pressure. This chalcogen serves, during the process time, aschalcogen source for the conversion of the metallic precursor layer intothe compound semiconductor layer. In addition or as an alternative tothis chalcogen coating, a chalcogen-containing gas can be fed in duringthe process time of the coating, for example a carrier gas containingchalcogen vapour. An inert gas such as nitrogen or a noble gas can beused as carrier gas in this instance.

The process time can in principle be interrupted by phases in which thetemperature of the coating is lower than 350° C. In such a case, theprocess time is composed of the sum of those times in which thetemperature of the coating is greater than or equal to 350° C.

Advantageously, the substrate is provided with a back contact. Forexample, a metal layer as back contact can be provided on a glasssubstrate, in particular a molybdenum layer. The back contact ispreferably divided into several strips by means of structuring, as aresult of which a series connection of various solar cell elements ofthe completed solar module can be realised. Said molybdenum coating doesnot represent a component of the metallic precursor layer or of thecoating in the sense of the present invention.

In principle, any substrates can be used in the method according to theinvention which are not adversely affected, while the method is beingcarried out, by the effect of heat or chemical reactions. Apart fromglass substrates, for example, strips of metal can thus also be used.

The metallic precursor layer can for example contain copper, indium andgallium and be applied using technologies of prior art, for example bymeans of sputtering. The metallic precursor layer can then in principleconsist of several metallic layers, for example firstly a layercontaining copper and gallium can be provided and a layer of indium canbe applied onto this. The metallic precursor layer can also be built upfrom several sub-layers of the same type, for example several layerscontaining copper and gallium. Repeating sequences of layers can also beprovided.

Using the method according to the invention, it is possible to producecompound semiconductor layers in which the elements involved aredistributed more homogeneously over the thickness of the compoundsemiconductor layer. This is illustrated by way of example in FIG. 2, inwhich, corresponding to the image in FIG. 1, the gallium-depthdistribution is shown in a CIGS compound semiconductor layer producedusing the method according to the invention. A comparison of FIGS. 1 and2 shows a clearly more homogeneous depth distribution of the gallium inthe CIGS compound semiconductor layer from FIG. 2. It would therefore bepossible to produce solar modules with significantly improved efficiencyfrom this compound semiconductor layer from FIG. 2.

The selection of a suitable activation barrier temperature depends onvarious parameters. So, for example, with greater availability ofchalcogen during the process time, even from an activation barriertemperature of 600° C., a comparatively homogeneous gallium distributionin a CIGS compound semiconductor layer can be realised. In practice anactivation barrier temperature of 640° C. has proven its value, and thisshould preferably be used.

In one advantageous variant embodiment of the invention, the coating, atleast during the process time, is arranged in a protective gasatmosphere. Preferably, the coating is already arranged in theprotective gas atmosphere during a heating phase preceding the processtime and this situation is maintained until the coating cools to anon-critical temperature. The arrangement of the coating in a protectivegas atmosphere serves, inter alia, to guarantee the least possibleoxygen partial pressure during a thermal treatment of the coating, inparticular during its conversion into a compound semiconductor layer,since the availability of oxygen could trigger undesirable chemicalreactions. This applies in the same way for hydrogen, when selenium isused. An atmosphere containing, for example, nitrogen or at least onenoble gas can be provided as protective gas atmosphere.

In one preferred variant embodiment of the method according to theinvention, before heating the coating to 350° C., a layer containing atleast one chalcogen, preferably sulphur or selenium, is deposited ontothe metallic precursor layer. The deposition is advantageously realisedby means of a physical deposition from the vapour phase at atmosphericpressure (APPVD). It is especially preferable for the chalcogendeposition to take place in a continuous process.

In one advantageous variant embodiment of the invention, the activationtime is selected to be shorter than 500 s, preferably shorter than 250s.

In one preferred variant embodiment of the method according to theinvention, the process time is selected to be shorter than 1200 s,preferably shorter than 600 s and especially preferably between 150 sand 500 s.

One variant embodiment of the method according to the invention makesprovision that the coating is heated to higher temperatures in severalsteps and then cooled. Preferably, this takes place in a segmentedcontinuous furnace, in the various segments of which the coating isbrought to various temperatures. 120 seconds has proven its value asdwell time in each segment. The heating of the coating can, however, inprinciple also take place in a conventional furnace. This can, however,be disadvantageous for industrial production, since this can only beused for batch operation.

In one variant embodiment of the invention, the substrate, if there isone, together with the back contact metallisation arranged thereon, isheated in the same way as the coating arranged on the substrate. Thismeans there are neither heating nor cooling installations present whichselectively heat or cool the coating more intensely than the substrate,nor are corresponding heating or cooling installations provided for thesubstrate, which heat or cool the substrate more intensely than thecoating. A cooling installation in this sense should also be understoodas a large, thermally inert mass, which, due to thermal coupling,manages to significantly delay the heating of the substrate or thecoating by comparison with the other part. In this variant embodiment ofthe invention, substrate and coating are thus essentially at the sametemperature.

One alternative variant embodiment of the method according to theinvention makes provision, on the other hand, that the coating, at leastduring a boost period, is kept at higher temperatures than thesubstrate. This keeping at higher temperatures is to be understood, inthis case, to mean that no defined temperature value is maintainedduring all or part of the boost period. Rather, the temperatures ofcoating and substrate can vary during the boost period. But at all timesduring the boost period, the temperature of the coating is always higherthan the temperature of the substrate. Preferably a temperaturedifference between coating and substrate of at least 30° C., especiallypreferably of at least 60° C., is provided.

Because the coating is kept at higher temperatures than the substrate,the thermal load on the substrate can be reduced. This extends the rangeof usable substrate materials.

Advantageously, the coating is kept at a higher temperature than thesubstrate, at least during the activation time, in order to minimise thethermal load on the latter.

It has been shown that advantageous results can be achieved even withboost periods of 15 s or less.

One refinement of the method according to the invention makes provisionthat, during the boost period, the temperature of the substrate is keptat values which are safe for the substrate, preferably using float glassas substrate and keeping its temperature to a value of less than 580° C.The word keeping in this case is to be understood to mean that thesubstrate temperature can also vary in principle, but always has valueswhich are safe for the substrate, so in the case of float glass alwayslies under 580° C. In this case, a safe temperature value is to beunderstood as a temperature at which no adverse modification of thesubstrate occurs, such as for example permanent modifications in a glassstructure as a result of significantly exceeding the fusion point, toogreat a thermal deformation or impairments due to chemical reactions.

In one variant embodiment of the method according to the invention, inorder to heat the coating more intensely than the substrate, at leastone lamp is used for illuminating the coating. It is preferable to usehalogen or xenon lamps for this. The wavelength range of theelectromagnetic radiation emitted by the lamps is advantageously adaptedto the absorption behaviour of the respective coating. In practice,lamps have proven useful which emit electromagnetic radiation withwavelengths mainly in the range of between 400 and 1200 nm.

In one advantageous variant embodiment, the substrate is reversed underthe electromagnetic radiation emitted by the lamps, in order to achievemore intense heating of the coating which is as homogeneous as possible.This can, for example, be realised by an oscillation movement of thesubstrates. If the coating, as described above, is heated in a segmentedcontinuous furnace, one refinement makes provision that the coating isilluminated with lamps while the substrate is being transported from onesegment to the next of the continuous furnace.

One advantageous variant embodiment of the method according to theinvention makes provision that during the process time, the substrate isarranged on a thermally inert carrier, preferably a graphite plate. Dueto the thermal inertia of the carrier, when there is sufficient thermalconduction between substrate and carrier, the coating can be heated,while the temperature of the substrate, due to the thermal coupling withthe carrier and the thermal inertia thereof, follows the temperature ofthe coating with a delay. If the method is carried out sufficientlyquickly, the coating can be brought to a comparatively high temperature,while the substrate reaches only lower temperatures. Before thesubstrate can reach the higher temperature of the coating, the methodand hence the heating process is already completed.

The device according to the invention for carrying out the method has afurnace chamber and at least one heating installation for heating thefurnace chamber. There is also at least one additional heatinginstallation provided for the selective heating of at least part of asystem brought into the furnace chamber. Said system consists of asubstrate and a coating arranged thereon. The word coating is to beunderstood here in the way explained above. A metal layer provided asback contact is to be assigned to the substrate.

The term selective heating is to be understood as meaning that theadditional heating installation essentially heats the at least one partof the system, the furnace chamber and the atmosphere prevailingtherein, however, only being slightly or indirectly heated by heatdissipation or heat abstraction from the heated at least part of thesystem.

The device according to the invention enables a comparatively rapid andenergy-efficient heating of the coating to the activation barriertemperature of at least 600° C.

In one variant embodiment of the device according to the invention, atleast one cooling installation is provided, by means of which at leastone part of the furnace chamber can be cooled.

According to a refinement of the device according to the invention, theat least one additional heating installation is arranged so as to heatthe coating of the system selectively. In this case, such selectiveheating of the coating is to be understood as meaning that the at leastone additional heating installation essentially heats the coating, whilethe furnace chamber and the atmosphere prevailing therein and thesubstrate, are, however, only slightly or indirectly heated by heatdissipation or heat abstraction from the heated coating. In this way thesubstrate can be kept at a lower temperature than the coating. Inparticular, the substrate temperature, as described above, can be keptat values which are safe for the substrate.

Preferably, the at least one additional heating installation is laid outin such a way that it can be used to bring the substrate and the coatingto temperatures which differ by at least 30° C., especially preferablyby at least 60° C.

In one preferred variant embodiment, at least one additional heatinginstallation is formed by at least one lamp. The word lamp means inprinciple any source of electromagnetic radiation which emitselectromagnetic radiation suitable for heating the coating present inthe respective application, or, in this case, the at least one part ofthe system consisting of coating and substrate. Lamps emitting radiationchiefly in the wavelength range of between 400 nm and 1200 nm haveproven useful for the compound semiconductor layers described above andtheir metallic precursor layers and also for chalcogens. For example,halogen or xenon lamps can be used.

In one variant embodiment of the device according to the invention, thefurnace chamber is made from graphite. The at least one heatinginstallation, for example consisting of an electrical resistance heater,and an optional cooling installation, for example consisting of a watercooler, are preferably embedded in the graphite walls. The at least onelamp is advantageously arranged in recesses in the graphite walls.

One refinement makes provision that the at least one lamp is arranged ina holder which is transparent for the radiation emitted by the lamp.This prevents any contamination of the furnace chamber in case the lampis destroyed. Servicing works can also be carried out with ease. Theholder need not necessarily be transparent for all radiation emitted. Inprinciple, transparency in one wavelength range is adequate, whichenables the desired selective heating in the respective application.Preferably the transparent holder is designed as a quartz tube.

In practice, twin tube emitters have proven useful as lamps, since thesehave greater mechanical stability.

In one advantageous variant embodiment, the at least one lamp isarranged behind a pane which homogenises the radiation emitted by the atleast one lamp. For example, a glass ceramic pane or a quartz glass panecan be provided.

One continuation of the invention provides for an exhaust gas channelfor extracting chalcogens not used in the conversion into the compoundsemiconductor layer out of the furnace chamber.

In one preferred variant embodiment of the device according to theinvention, this is designed as a continuous furnace and the furnacechamber is divided into several segments. Different temperatures can bedeveloped in the various segments. This is done by means of the at leastone heating installation and, if there is one, the at least one coolinginstallation. A transport device is advantageously provided to transportthe substrates from one segment to the next, for example a push rod,which has proven useful, in particular when the substrates haveidentical dwell times in all the segments. In this embodiment, anadditional heating installation can be provided, for example within onesegment, or additional heating installations can be arranged in severalsegments.

Preferably, the several segments are thermally insulated from eachother, so that significant temperature differentials can be realisedbetween adjacent segments. Additionally, the individual segments caneach be thermally insulated against the environment of the device.

Advantageously, a protective gas atmosphere can be developed in thefurnace chamber. Gas inlet and outlet lines of prior art can be providedaccordingly. In the case of a continuous furnace, gas locks are to bearranged at loading and discharge openings, for example gas curtains ofprior art. This allows the substrates to be arranged in a protective gasatmosphere.

In one variant embodiment of the device according to the invention, atleast one additional heating installation is arranged between twoconsecutive segments of the continuous furnace. In this way the at leastone part of the system, in particular the coating, can be heated duringtransport of the substrate from one segment to the next. As a result, inparticular, any interference with the temperature in the segments by theat least one additional heating installation can be minimised.

One advantageous variant embodiment of the device according to theinvention makes provision that the furnace chamber is encased by ahousing and a space is formed between furnace chamber and housing, whichhas at least one flushing gas inlet and at least one extraction channel.The space formed can be flooded with a flushing gas and this can besuctioned out.

This can reduce the extent of penetration of gases from the environmentinto the furnace chamber and any escape of process gases from thefurnace chamber into the environment. The protective gas atmosphere inthe furnace chamber can thus be improved. The housing is in this casepreferably designed as a stainless steel housing.

In principle, it is not necessary for the flushing gas introduced intothe space to be extracted via the extraction channel. Instead, asufficiently large influx of flushing gas can be provided so that theextraction channel simultaneously acts as a flushing gas outlet.

One refinement of the device according to the invention makes provisionthat at least one sensor from the group consisting of an oxygen sensorand a hydrogen selenide sensor is arranged in the space formed betweenfurnace chamber and housing.

In one advantageous variant embodiment the housing is designed to becoolable, so that heat given off by the furnace chamber can beabstracted.

One refinement of the device according to the invention makes provisionfor a reversing device. This is arranged so as to reverse substrateswhile they are being heated by the at least one additional heatinginstallation. The substrates are thereby moved around in the area ofinfluence of the additional heating installation, in order tocompensate, at least in part, for inhomogeneities, which in the case ofa lamp are radiation inhomogeneities. Preferably the reversing device isdesigned as an oscillation device, so that the substrates can bedisplaced in oscillating movements.

The invention will next be explained in more detail with the aid of thefigures. So far as useful, the elements with the same action areprovided with the same reference numbers. The figures show:

FIG. 1 gallium-depth distribution in a CIGS compound semiconductor layerproduced according to the state of the art.

FIG. 2 gallium-depth distribution in a CIGS compound semiconductor layerwhich was produced by a method according to the invention.

FIG. 3 basic diagram of a first embodiment of the method according tothe invention.

FIG. 4 schematic view of an embodiment of the device according to theinvention and illustration of a further embodiment of the methodaccording to the invention.

FIG. 5 time-temperature profile of a substrate in a further embodimentof the method according to the invention in which the substrate isheated in the same way as the coating arranged thereon.

FIG. 6 snapshot in schematic view of a coating arranged on a substrate,which has already run through part of the process time, but has not yetbeen heated to the activation barrier temperature or beyond.

FIG. 7 result of an X-ray diffraction measurement according toBragg-Brentano on the layer structure shown in FIG. 6.

FIG. 8 results from measurements conducted at various temperaturesduring the heating of the coating of the angular-dependent intensity ofthe diffracted X-radiation.

FIG. 9 basic diagram of the time-temperature profile of the coatingduring the application of a further embodiment of the method accordingto the invention, in which the coating is heated during a boost periodto a higher temperature than the substrate.

FIG. 10 schematic view of the arrangement of the substrate on athermally inert carrier according to one variant embodiment of themethod according to the invention.

As already explained above, FIG. 1 shows a gallium-depth distribution ina CIGS compound semiconductor layer produced using a method according tothe state of the art. This has been produced with a deposition-reactionprocess at a maximum temperature of the coating of 520° C. A solar cellproduced from this compound semiconductor layer has an efficiency of8.3% and a open circuit voltage of 460 mV.

In contrast hereto, FIG. 2 shows a gallium-depth distribution in a CIGScompound semiconductor layer produced using the method according to theinvention under a maximum temperature of 640° C. The more homogeneousdistribution of the gallium is clearly visible. The solar cell producedfrom the manufactured compound semiconductor layer shows, at 13.6%, asignificantly higher efficiency. As expected, there also arises a higheropen circuit voltage, in the amount of 600 mV.

FIG. 3 shows a basic diagram of a first embodiment of the methodaccording to the invention. According to this, firstly a metallicprecursor layer is sputtered onto a substrate 80. In this case, forexample, firstly copper and gallium can be sputtered on together and, ina further deposition process, indium can be sputtered on. A multipleapplication of a sequence of layers of this or another type is possiblein principle. The substrate used is also provided with a metallic backcontact coating, on which the layers which are sputtered on come to lie.Here, for example, a molybdenum layer can be used as metallic backcontact layer. As described above, this back contact layer isadvantageously already structured.

Subsequently, selenium is vapour-deposited at atmospheric pressure ontothe metallic precursor layer and thus is pre-capitated thereon 82. Thisis followed by a heating phase. Also, the coating initially formed frommetallic precursor layer and deposited selenium layer is kept attemperatures of over 350° C. 84 and at the same time the metallicprecursor layer is converted, using selenium from the deposited 82selenium layer, into a CIGS compound semiconductor layer 84. While thecoating is kept at a temperature of over 350° C. 84, i.e. during theprocess time, the coating is heated to an activation barrier temperatureof 640° C. and kept for an activation time at the activation barriertemperature of 640° C. 86. In other variant embodiments, the temperatureof the coating may exceed the activation barrier temperature during theactivation time.

FIG. 4 shows an embodiment of a device according to the invention, whichfor example can be used to carry out the method according to theinvention in accordance with FIG. 3. The device is designed as acontinuous furnace 24 and has a furnace chamber 1, the walls of whichare made of graphite or are clad with graphite. The furnace chamber 1 isfurthermore divided into several segments S1, S2, S3, S4, S5, S6. Ineach of these segments S1, S2, S3, S4, S5, S6, by means of the heatinginstallations 8 arranged in segments S1 to S4 and the coolinginstallations 9 arranged in segments S5 and S6, a coating passingthrough the furnace chamber 1 can be brought to a predeterminedtemperature. The heating 8 and cooling installations 9 are preferablyembedded in the graphite walls of the furnace chamber 1. The segmentsS1, S2, S3, S4, S5, S6 are thermally insulated from each other and alsoeach is designed to be thermally insulated from the environment. For thesake of greater clarity, the associated means of insulation are notshown in FIG. 4. Due to the thermal insulation described, significantlydifferent temperatures can be realised in the various segments S1, S2,S3, S4, S5, S6 and energy expenditure can be reduced.

In segment S1, a loading opening 2 is provided via which substrates canbe introduced into the furnace chamber 1. Accordingly, segment S6 has adischarge opening 3, via which the substrates can be guided out of thefurnace chamber 1. A protective gas atmosphere can be developed in thefurnace chamber 1. To this end, and to prevent any penetration ofdamaging gases, for example gases containing oxygen or hydrogen, intothe furnace chamber 1, gas curtains 4, 5 are arranged at the loadingopening 2 and at the discharge opening 3. Furthermore, FIG. 4 shows anexhaust gas channel 12 for the extradtion of selenium or otherchalcogens not used in the conversion into the compound semiconductorlayer out of the furnace chamber 1.

Inside the furnace chamber, a transport device 7 is arranged, by meansof which substrates 6 can be transported through the furnace chamber 1.A push rod may be provided as such a transport device 7, for example.

In order to further improve the protective gas atmosphere in the furnacechamber 1, this is provided with a housing 13, which in the present caseis designed as a stainless steel housing 13. Corresponding to theloading opening 2 and discharge opening 3 of the furnace chamber 1, thishousing has a loading opening 14 and a discharge opening 15. Thestainless steel housing 13 is also provided with a flushing gas inlet 17and with an extraction channel 16. A space 23 formed between the furnacechamber 1 and the stainless steel housing 13 can be flushed with aprotective gas, for example nitrogen, via this flushing gas inlet 17 andthe extraction channel 16. An oxygen sensor 25 a and a hydrogen selenidesensor 25 b are arranged in the space 23. This allows any penetration ofoxygen or hydrogen into the space 23 to be detected.

FIG. 4 further illustrates an embodiment of a method according to theinvention. In this one, first of all a substrate 6 provided with amolybdenum backside contact, on which a metallic precursor layer 18 isapplied, for example by means of sputtering 80, and which has also beenprovided with a selenium layer 19, for example with an APPVD-deposition,is introduced into the furnace chamber 1 via the loading opening 2. Oncein segment S1, where the oxygen partial pressure is low because of theprotective gas atmosphere, it is heated to about 150° C. Following afurther transport into segment S2, it is heated to about 550° C., sothat at least a partial conversion of the metallic precursor layer 18with selenium from the selenium layer 19 into a CIGS compoundsemiconductor layer 20 takes place. Once the coating 18, 19, initiallyformed from metallic precursor layer 18 and selenium layer 19, attains atemperature of 350° C., the duration of the process time commences, asthe further heating, beyond 350° C. to 550° C., represents a part ofkeeping the coating 18, 19 at temperatures of at least 350° C.

Next, the substrate is introduced into segment S3, in which thesubstrate and hence also the coating is heated to 650° C. and thusbeyond an activation barrier temperature of 640° C. used for thisembodiment. The activation time begins on reaching the activationbarrier temperature. Next, in segment S3, the coating temperature of650° C. is maintained, until the substrate is transported further, intosegment S4. There a cooling of the substrate takes place, and also ofthe coating formed by the CIGS compound semiconductor layer, ultimatelyto about 600° C. Once the temperature falls below the activation barriertemperature of 640° C., the activation time ends. Unconverted seleniumis passed out of segment S4 via the exhaust gas channel 12 out of thefurnace chamber 1.

In the adjacent segments S5 and S6, the substrate together with itscoating are cooled down with the aid of the cooling installations 9,firstly to about 450° C. and then 300° C., before it is ejected from thecontinuous furnace 24 via the discharge openings 3 and 15.

In the embodiment described, it has proven useful to leave the substratein each segment for about 120 seconds. Furthermore, a process time ofabout 480 seconds has proven useful, during which the substrate and thusalso the coating formed from the metallic precursor layer 18 and theselenium layer 19 and/or the CIGS compound semiconductor layer 20 iskept at temperatures of at least 350° C. As activation time, at whichsubstrate 6 and coating 18, 19, 20 are kept at temperatures greater thanor equal to the activation barrier temperature of 640° C., a value of120 seconds has proven useful.

Lamps 10 arranged in the furnace chamber 1 and in segment S3 therein forirradiating the coating of the substrate 6 are not used in theembodiment of the method according to the invention just described. Sothe substrate as well as the coating are heated in the same way, withthe result that they have roughly the same temperature.

In another variant embodiment of the method according to the invention,in which the coating is kept at higher temperatures than the substrateduring a boost period, however, the lamps 10 provided can be put to usefor this purpose. These lamps 10 are preferably arranged in recesses inthe wall of the furnace chamber 1 and each encased by a quartz claddingglass 10, which, in the event of a lamp 10 exploding, enables this to bechanged quickly and easily and prevents contamination of the furnacechamber 1. In order to achieve the most homogeneous possible irradiationof the coating 18, 19, 20 by means of the lamps 10, a glass ceramic pane11 is provided between these lamps 10 and the substrate 6.

The transport device 7 also has a reversing device 7 a, which in thiscase is designed as an oscillation device 7 a and by means of which, insegment S3, the substrate 6 can be oscillated under the glass ceramicpane 11, in order to enable a homogeneous irradiation of the coating 20with the electromagnetic radiation emitted by the lamps 10.

In the embodiment of the method according to the invention described inconnection with FIG. 4, selenium from the selenium layer 19 is madeavailable as chalcogen for the conversion of the metallic precursorlayer 18 into the CIGS compound semiconductor layer 20. Instead of or inaddition to this, in another variant embodiment of the method accordingto the invention, a chalcogen, in particular selenium, can be madeavailable for the conversion via a selenium vapour feed 22 into thefurnace chamber 1. Via this selenium vapour feed 22, for example,selenium in vapour form can be introduced by means of a carrier gas intothe furnace chamber 1.

FIG. 5 shows the time-temperature profile of coating and substrate in afurther embodiment of the method according to the invention. In thisone, again, a layer containing copper and gallium as well as an indiumlayer is sputtered onto a glass substrate provided with a molybdenumback contact and a selenium layer is deposited by means of APPVD. Theheating of the substrate and the coating takes place, not in acontinuous furnace, but in a conventional furnace under a nitrogenprotective gas atmosphere at approximately atmospheric ambient pressure.As can be deduced from FIG. 5, the substrate and its coating are firstlyheated, with a temperature increase of about 6° C./s, to a temperatureof about 500° C., before the substrate is brought, at a heating-up rateof about 1° C./s, to a final temperature of 700° C. and is kept at thistemperature value for about 30 s. There then follows a cooling of thesubstrate. A temperature of 640° C. is selected as activation barriertemperature Tg. So the process time tp, as represented in FIG. 5, isabout 450 s, and the activation time tg about 140 s.

FIGS. 6 to 8 illustrate the fact that the method according to theinvention enables a more homogeneous gallium distribution in thecompound semiconductor layer produced and that temperature influenceshomogeneity. FIG. 6 shows a snapshot of a substrate provided with acoating during the method according to the invention before reaching theactivation barrier temperature. In this moment, similarly to CIGScompound semiconductor layers produced according to the state of theart, there is a gallium-rich and indium-poor CIGS layer 30 close to thesubstrate, and/or the back contact arranged on the substrate 6 and notillustrated. Above this is a comparatively gallium-poor CIGS layer 32,which correspondingly contains more indium.

FIG. 7 shows the result of an X-ray diffraction measurement according toBragg-Brentano on a layer structure similar to that shown in FIG. 6. The220/204-reflection of the chalcopyrite structure is visible. The peakwith a maximum at about 44.5° results from diffracting the X-rays on thegallium-poor CIGS layer 32. On the right flank of this peak can be seenthe superposition with a second, significantly weaker signal, whichresults from diffracting the X-rays on the gallium-rich CIGS layer 30.This is due to the fact that a higher gallium proportion in a CIGS alloyleads to a smaller lattice constant and hence to a greater diffractionangle. The intensity of the radiation diffracted on the gallium-richCIGS layer 30 is reduced, because of the lesser layer thickness,indicated schematically in FIG. 6, of the gallium-rich CIGS layer 30 andbecause of X-ray absorption effects in the thicker gallium-poor CIGSlayer 32, by comparison with the intensity of the peak of thegallium-poor CIGS layer 32.

Corresponding X-ray diffraction measurements on a substrate treatedaccording to the embodiment from FIG. 5 were conducted at various pointsin time during the process, which in FIG. 5 are characterised by, thesubstrate and coating temperature prevailing at the respective point intime. The results of these X-ray diffraction measurements are reproducedin FIG. 8, where the graphs of the individual measurements arevertically displaced from each other for the purpose of easiercomparability.

The peaks with maxima at about 40.4° represent a 110-reflection of themolybdenum of the back contact arranged on the substrate. As can be seenfrom FIG. 8, this reflection does not undergo any substantialmodification during the thermal treatment.

The graph reflecting the measurement results on attaining a substrateand/or coating temperature of 515° C. has a peak with a maximum at about44°. This corresponds to the 220/224-reflection of copper indiumselenide or a very gallium-poor CIGS layer, where the transitions arefluid. The corresponding peak of the graph belonging to a substrateand/or coating temperature of 600° C. already reveals on its right flanka superimposed, further peak, representing the 220/204-reflection of agallium-rich CIGS layer, as has already been discussed in connectionwith FIGS. 6 and 7. The compound semiconductor layer thus consists, atthe time of measurement of the 600° C. graph, of a very gallium-poorCIGS layer (peak at approx. 44°) and a CIGS layer with a high proportionof gallium.

At a temperature of 640° C. the superimposed further peak is morestrongly marked and has shifted towards smaller angles, the peak atabout 44° appears weaker. This indicates an exchange of indium from thegallium-poor CIGS layer with gallium from the gallium-rich CIGS layer.Furthermore, the peak at approx. 44° begins to shift towards largerangles, which indicates that gallium is diffused on the upper side ofthe gallium-poor CIGS layer, which has a positive effect on open circuitvoltage and efficiency of a solar module and/or a solar cell produced.The shift trends described stand out even more clearly in the graphbelonging to a substrate- and coating temperature of 680° C.

FIG. 8 thus illustrates that in the embodiment examined, by keeping thecoating at or above an activation barrier temperature of 640° C. for acertain time, a CIGS compound semiconductor layer with significantlyimproved homogeneity over the thickness of the compound semiconductorlayer can be manufactured. As already explained above, furtherinvestigations have revealed that, depending on how the method iscarried out, even activation barrier temperatures of 600° C. can lead toan improved homogeneity of the compound semiconductor layersmanufactured.

FIG. 9 illustrates, with the aid of the time-temperature profile of thecoating, in a simplified schematic view, a further embodiment of themethod according to the invention, which can, for example, be carriedout with the continuous furnace shown in FIG. 4 using the lamps 10arranged in the continuous furnace 24. It is therefore described by wayof example, making reference to the illustration in FIG. 4.

In this embodiment, a float glass substrate 6 provided with a molybdenumback contact and coated with a metallic precursor layer 18 and also anAPPVD-selenium layer 19 is introduced via the loading openings 2, 14into the first segment S1 of the furnace chamber 1, where it is heated,together with the coating composed of metallic precursor layer 18 andselenium layer 19, to 150° C. It is next moved on to segment S2 where,together with coating 18, 19, 20, it is heated to a temperature of about500° C., before being placed in segment S3. Once the coating attains atemperature of 350° C. in segment S2, the duration of the process timecommences. Substrate 6 and coating 18, 19, 20 are firstly heatedtogether in segment S3 to a temperature of 550° C. Until this point, thetemperature profiles of substrate 6 and coating 18, 19, 20 areessentially identical. Next, however, the coating 20 (the conversion ofthe metallic precursor layer 18 and the selenium layer 19 has largelyalready taken place in the meantime) is heated by means of the lamps 10,as indicated in FIG. 9, more intensely than the substrate 6 and as aresult is kept at higher temperatures during a boost period tb than thesubstrate 6. As a result of the more intense heating by the lamps 10,the coating 20 attains a maximum temperature of about 650° C. andthereby, exceeds the activation barrier temperature of 640° C. used inthis embodiment. Consequently, the coating 20 is kept at highertemperatures throughout the entire activation time tg than thesubstrate. The boost period tb in the embodiment in FIG. 9 is longerthan the activation time tg.

The boost period tb is selected to be comparatively brief, preferably 15s. During this time, the glass substrate is unable to follow thetemperature increase of the coating and remains at a safe temperaturefor the glass substrate 6 of under 580° C. The coating cools down againcorrespondingly quickly after switching off the lamps 10, to 550° C.Both substrate 6 and coating 20 are then transported into segments S4 toS6, where they are successively cooled down; in segment S4 to about 450°C., in segment S5 to about 350° C. and in segment S6, finally, to about200° C. As dwell time in each of the segments S1, S2, S3, S4, S5, S6,for example, 120 s may be provided.

In order to enable a homogeneous illumination of the coating 20 by meansof the lamps 10, the transport device 7 of the continuous furnace 24, asdescribed above, is designed to set the substrate, at least in segmentS3, in reverse movements, for example in oscillation, in order to ensurea homogeneous illumination of the coating by means of the lamps 10.

The lamps 10 can in principle also be arranged at a location other thanin segment S3, for example between individual segments. The coatingcould be illuminated in this way while the substrate is beingtransported from one segment to the next. Furthermore, a combination oflamps 10 arranged in segment S3, or in other segments, and lampsarranged between adjacent segments is also possible.

In another variant embodiment of the method according to the invention,the substrate is arranged on a thermally inert carrier, i.e. a carrierwith relatively great thermal mass, in order to keep the substrate andthe coating at different temperatures. To this end, as shownschematically in FIG. 10, the substrate 6 is arranged 88 on a thermallyinert carrier material, in this case a graphite plate 26. Because of theresulting thermal coupling between substrate 6 and graphite plate 26,the substrate 6 is less able to follow any heating of the coating, oreven heats up only with a delay. This can be exploited to heat thecoating 18, 19, 20 to higher temperatures, and/or to keep it at highertemperatures, than the substrate 6. The arrangement 88 described of thesubstrate 6 on a thermally inert carrier, in particular a graphite plate26, can take place as an alternative or in addition to the describedheating of the coating by means of lamps. In particular, the substrate 6can run through the continuous furnace 24 on the graphite plate 26 fromFIG. 4.

LIST OF REFERENCE NUMBERS

1 furnace chamber

2 loading opening

3 discharge opening

4 gas curtain

5 gas curtain

6 substrate with molybdenum backside contact

7 transport device

7 a oscillation device

8 heating installation

9 cooling installation

10 lamp in quartz tube

11 glass ceramic plate

12 exhaust gas channel

13 stainless steel housing

14 loading opening

15 discharge opening

16 extraction channel

17 flushing gas inlet

18 metallic precursor layer

19 selenium layer

20 CIGS compound semiconductor layer

22 selenium vapour feed

23 space

24 continuous furnace

25 a oxygen sensor

25 b hydrogen selenide sensor

26 graphite plate

30 gallium-rich CIGS layer

32 gallium-poor CIGS layer

80 sputtering on of metallic precursor layer

82 APPVD-selenium deposition

84 heating and keeping coating at temperature and conversion intocompound semiconductor layer

86 keeping coating at activation barrier temperature

88 arranging substrate on graphite plate

S1 first segment of the continuous furnace

S2 second segment of the continuous furnace

S3 third segment of the continuous furnace

S4 fourth segment of the continuous furnace

S5 fifth segment of the continuous furnace

S6 sixth segment of the continuous furnace

Tg activation barrier temperature

tg activation time

tp process time

tb boost period

1-22. (canceled) 23: A method for producing a I-III-VI compoundsemiconductor layer, which comprises the steps of: providing a substratewith a coating having a metallic precursor layer; keeping the coating,for a duration of a process time, at temperatures of at least 350° C.;during the process time, converting the metallic precursor layer in apresence of a chalcogen at an ambient pressure of between 500 mbar and1,500 mbar into the I-III-VI compound semiconductor layer; and keepingthe coating, for a duration of an activation time, at temperatures whichattain at least an activation barrier temperature, whereby a value of atleast 600° C. is selected as the activation barrier temperature. 24: Themethod according to claim 23, which further comprises selecting thevalue of the activation barrier temperature to be 640° C. 25: The methodaccording to claim 23, which further comprises disposing the coating, atleast during the process time, in a protective gas atmosphere. 26: Themethod according to claim 23, which further comprises selecting theactivation time to be less than 500 seconds. 27: The method according toclaim 23, which further comprises selecting the process time to be lessthan 1,200 seconds. 28: The method according to claim 23, which furthercomprises heating the coating in a segmented continuous furnace and indifferent segments of the segmented continuous furnace the coating isbrought to different temperatures. 29: The method according to claim 23,which further comprises converting the metallic precursor layer into acopper-indium-selenide or a copper-indium-gallium-selenide compoundsemiconductor layer. 30: The method according to claim 28, which furthercomprises keeping the coating at higher temperatures than the substrateduring a boost period. 31: The method according to claim 30, whichfurther comprises keeping the coating, at least during the activationtime, at higher temperatures than the substrate. 32: The methodaccording to claim 30, which further comprises keeping a temperature ofthe substrate, during the boost period, at values which are safe for thesubstrate. 33: The method according to claim 30, which further comprisesilluminating the coating with at least one lamp for more intense heatingof the coating compared with the substrate. 34: The method according toclaim 33, which further comprises illuminating the coating with the atleast one lamp, while the substrate is being transported from onesegment of the continuous furnace into a next segment of the continuousfurnace. 35: The method according to claim 23, which further comprisesduring the process time disposing the substrate on a thermally inertcarrier. 36: The method according to claim 23, which further comprisesdisposing the coating, at least during the process time, in a protectivegas atmosphere containing one of nitrogen gas or a noble gas. 37: Themethod according to claim 23, which further comprises selecting theactivation time to be less than 250 seconds. 38: The method according toclaim 23, which further comprises selecting the process time to be lessthan 600 seconds. 39: The method according to claim 23, which furthercomprises selecting the process time to be between 150 s and 500seconds. 40: The method according to claim 32, which further comprisesforming the substrate from float glass and keeping the temperature ofthe float glass at values of less than 580° C. 41: The method accordingto claim 23, which further comprises during the process time disposingthe substrate on a graphite plate. 42: A device for producing a I-III-VIcompound semiconductor layer, the device comprising: a furnace chamber;at least one first heating installation for heating said furnacechamber; and at least one second heating installation for selectiveheating of at least part of a system introduced into said furnacechamber, the system containing a substrate and a coating disposed on thesubstrate. 43: The device according to claim 42, wherein said at leastone second heating installation is disposed for a selective heating ofthe coating. 44: The device according to claim 42, wherein said at leastone second heating installation is one of a plurality of second heatinginstallations, at least one of said second heating installations has atleast one lamp for irradiating the at least part of the system. 45: Thedevice according to claim 44, wherein said at least one second heatinginstallation further contains a pane, wherein said at least one lamp isdisposed behind said pane which homogenizes radiation emitted by said atleast one lamp. 46: The device according to claim 42, wherein the deviceis configured as a continuous furnace having said furnace chamber beingdivided into several segments, whereby different temperatures can bedeveloped in said different segments and whereby said several segmentsare thermally insulated from each other. 47: The device according toclaim 46, wherein said at least one second heating installation is oneof a plurality of second heating installations and at least one of saidsecond heating installations is disposed within one of said segments ofsaid continuous furnace. 48: The device according to claim 46, whereinsaid at least one second heating installation is one of a plurality ofsecond heating installations and at least one of said second heatinginstallations is disposed between two consecutive ones of said segmentsof said continuous furnace. 49: The device according to claim 42,further comprising: a housing encasing said furnace chamber and a spaceis formed between said furnace chamber and said housing; at least oneflushing gas inlet supported by said housing; and at least oneextraction channel supported by said housing. 50: The device accordingto claim 44, further comprising a reversing device for reversing thesubstrates while they are being heated by means of said at least onelamp. 51: The device according to claim 45, wherein said pane isselected from the group consisting of a glass ceramic pane and a quartzglass pane. 52: The device according to claim 49, wherein said housingis a stainless steel housing. 53: The device according to claim 50,wherein said reversing device is an oscillation device. 54: The deviceaccording to claim 44, wherein said one second heating installation hasa holder and said at least one lamp is disposed in said holder which istransparent for light emitted by said lamp, and is configured as aquartz tube.